SGM3122 Datasheet, PDF (14/17 Pages) SG Micro Corp – PWM Brightness Control

SGM3122
White LED Charge Pump Current
Source with PWM Brightness Control
APPLICATION INFORMATION
Typical Application Using 3 White LEDs and 6 Green LEDs for LCD Backlight and Keyboard
Lighting
The SGM3122 can be used to power any kind of LED. It is also possible to mix white LEDs with color LEDs which have
a lower forward voltage. The LED with the highest forward voltage (typically the white LED) has to be connected to D1,
because the output voltage of the charge pump is regulated in such a way to keep the voltage drop from D1 to GND at
400mV (with EN1 = EN2 = 1). Therefore the output voltage of the charge pump is regulated to:
V_OUT = V_D1 + V_FLEDD1
V_OUT — Output voltage at VOUT
V_D1 — Voltage from D1 to GND (V_SOURCE at D1 pin, see electrical characteristics)
V_FLEDD1 — Forward voltage of the LED connected to D1.
Resistor R_g is used to provide current sharing between the 6 green LEDs. The upper value is calculated using:
R_g
=
V_FLEDD1
l_g

V_Fg
V_Fg — Forward voltage of a green LED
I_g — Current per green LED
C2
0.47µF
C1
0.47µF
Li-ion
CIN
Battery
1µF
C2- C1- C1+ C2+
VIN
VOUT
D1
EN1 SGM3122 _D2
EN2
D3
D4
ISET GND PGND
5.1kΩ
1µF3 _{White LEDs}
with 30mA Each
R_g = 220Ω
6 Green LEDs
with 5mA Each
Set Current to 30mA
Per Current Sink
(with EN2 = EN1 = 1)
SG Micro Corp
14
www.sg-micro.com